A major issue for security service providers is to be able to have confidence in the integrity of the monitoring systems at their disposal. They require reliable systems that are rugged and can operate effectively for years of field operation, and yet are not prone to false alarms under a wide variety of operational and environmental conditions.
Intrusion detection systems are widely employed to secure a large variety of sites, from low-security private residences to high-security military installations. Most of the systems available comprise a physical barrier and an electronic detection capability.
The most widely used conventional systems utilise the following technologies:                CCTV cameras        Taut-wire fences        Leaky wave coax cables        E-field sensors        Microphonic cables        Strain gauged systems        X-band line-of-sight radar beams        Free-space infrared surveillance equipment        
A major limitation for many conventional perimeter security systems is their susceptibility to electromagnetic interference and their inability to operate reliably over long distances. Furthermore, their costs usually increase significantly as the length of a protected perimeter increases.
Traditional perimeter security systems attempt to overcome their distance limitations through the use of multiple, contiguous zones covering the full extent of a perimeter. Generally, this zoning can assist in supporting other distance-limited security devices such as video cameras and lighting for monitoring suspected breach attempts.
In many cases, with traditional systems, these zones can be limited in length to as low as one to three hundred meters. With industrial sites often having perimeters in excess of two kilometres, there may be a requirement for at least six to twenty zones in such cases. Government and military sites can be considerably larger.
Furthermore, with traditional systems there is generally a need to install zone controller electronics each time a new zone is required. Consequently, for systems with large numbers of zones the cost can become prohibitive. In addition, there is a significant increase in reliability issues and potential maintenance costs as the incidence of perimeter-mounted electronics increases. In particular, lightning strikes, a common occurrence for steel fences, can easily disable many zones in the one hit where external electronics are involved.
All these limitations of active perimeter monitoring systems can be overcome with an optical fibre based sensing system. The inventions disclosed in this specification relate to a method and systems formed for monitoring a perimeter barrier against intrusion or tampering utilizing fibre optic sensing technology.
This is possible because optical fibres can be more than mere signal carriers. Light that is launched into and confined to the fibre core propagates along the length of the fibre unperturbed unless acted upon by an external influence. In a sensing application, the optical fibre should be installed such that the disturbing influence is coupled from the structure of interest to the fibre, thus altering some characteristic of the light within the fibre.
Specialised sensing instrumentation may be configured such that any disturbance of the fibre which alters some of the characteristics of the guided light (ie., amplitude, phase, wavelength, polarisation, modal distribution and time-of-flight) can be monitored, and related to the magnitude of the disturbing influence. Such modulation of the light makes possible the measurement of a wide range of events and conditions, including:                strain        displacement        cracking        vibration/frequency        impact        acoustic emission        temperature        load        
Fibre optic sensor (FOS) technology has progressed at a rapid pace over the last decade. Different configurations of fibre sensing devices have been developed for monitoring specific parameters, each differing by the principle of light modulation. Fibre optic sensors may be intrinsic or extrinsic, depending on whether the fibre is the sensing element or the information carrier, respectively. They are designated “point” sensors when the sensing gauge length is localised to discrete regions. If the sensor is capable of sensing a measured field continuously over its entire length, it is known as a “distributed” sensor; “Quasi-distributed” sensors utilise point sensors at various locations along the fibre length. Fibre optic sensors can be transmissive or can be used in a reflective configuration by mirroring the fibre end-face.
Hence, fibre optic sensors are actually a class of sensing device. They are not limited to a single configuration and operation unlike many conventional sensors such as electrical strain gauges and piezoelectric transducers.
Furthermore, FOS technology has many advantages over conventional sensing devices because of its high resolution and its ability to work in real-time, without electromagnetic interference problems. Furthermore, sensor lengths can vary between different devices; from point sensing configurations to very long sensing configurations (over 50 km long). In addition, they are made from a very durable material that is corrosion resistant (pure silica).
Consequently, fibres are now replacing the role of conventional electrical devices in sensing applications to the extent where we are now seeing a multitude of sensing techniques and applications being explored for practical gain, including in the perimeter security field. Using the latest technology in fibre optic sensing it is now possible to secure many types of perimeters, fences and barriers.
Fibre optic cables, when used as sensors, can be applied to fences, walls, rooftops, or air-conditioning ducts, or they can be buried in gravel or under lawns. They can be used for the protection of buried pipelines, prisons, government buildings, defence sites, chemical laboratories, power plants, pumping stations, embassies, airports, secure residential complexes, manufacturing plants, storage facilities, communications facilities, harbours and even international borders.
One particular benefit of fibre optic based systems is their immunity to electrical interference, particularly important for installations near high voltage electrical equipment, high power radio transmissions or in areas subject to lightening strikes.
As a result, considerable research has been underway over the past decade into the development of fibre optic perimeter monitoring systems. Previous research in this area involved the use of the following fibre optic sensing techniques:
1. Bistable Techniques:
The bistable fibre optic sensor is the simplest form of sensor, detecting damage or other interruption by the absence of light in a fibre. This technique usually requires the physical fracture of the fibre which is detected by a photodiode as an intensity loss or null.
2. Optical Time Domain Reflectometry (OTDR)
Techniques:
The basis of OTDR is essentially optical radar. A narrow optical pulse from a laser is launched into a multimode (usually) fibre and the light backscattered due to optical inhomogeneities is used to determine the attenuation properties of the optical fibre along it's entire length. The attenuation is characterised by analysing the time dependence of the detected Rayleigh backscattered light.
OTDR techniques allow for distributed sensing and are capable of detecting stress, strain, temperature, electric and magnetic fields, and mechanical faults along the entire length of the fibre. OTDR can be used to detect and locate breaks in a fibre due to Fresnel reflection at the fracture. OTDR can be a very useful tool for detecting and locating the above listed parameters but the long signal integration times needed to obtain reasonable signal-to-noise ratios limits this technique to detecting permanent, usually destructive, effects on the fibre cable.
3. Modalmetric Multimode Techniques:
This optical fibre sensing technique is based on the modulation in the distribution of modal energy propagated in a fibre. Although this type of sensor can be effective, the modulation of the modal pattern is generally non-linearly related to all disturbances, resulting in deep fading and drifting of the output signal. This behaviour generally limits the use of this sensor for quantitative strain measurements, but nonetheless it can be used as a threshold-type sensor. Modalmetric sensors are capable of sensing many parameters, however, their sensitivities are generally lower than interferometric sensors and localisation of the sensing region is difficult (resulting in sensitive leads). However, for security applications the modalmetric sensors offer the advantage of detecting disturbances over long lengths of fibre (they are generally a distributed sensor).
However, in 1994 the present applicant developed a novel distributed fibre optic vibration sensing technology (see PCT specification PCT/AU95/00568). The sensing technique was based on a unique fibre optic modalmetric sensor configuration. This sensor provides a simple, effective and inexpensive technique to detect and characterise both small and large, static and dynamic disturbances on any optical fibre cable, anywhere along its entire length in a non-intrusive way, directly and in real-time. This sensing technique is based on the modulation of the modal distribution in a multimode optical fibre by external disturbances. This technique overcomes the inherent weaknesses of most multimode fibre optic sensors, offering mechanically stable and linear sensing. In this method, the sensor response is a direct function of the disturbance on the sensitised portion of the fibre, regardless of where the disturbance occurs along the length. The disturbance may be in the form of physical movement (ie., compression (radially or axially), elongation, twisting, vibration, etc.) or microphonic effects (ie., travelling stress waves or acoustic emissions). This sensor had a further advantage over most other modalmetric sensors in that it can operate as a single-ended device by mirroring the fibre end-face.
4. Periodic Microbending Techniques:
In this technique, when a fibre is bent the light propagating in the core is coupled into the cladding and lost. The smaller the radius of curvature of the bend the higher the loss of radiation. This principle is the basis of the periodic microbend sensor. Thus, the transmission of the optical fibre is reduced by applying a periodic force on the fibre. Maximum transmission loss occurs when the bending is applied periodically with a specific bend pitch. Consequently, this technique requires a specially designed clamp to apply pressure to the fibre at the point of interest. Therefore, it is not a distributed technique, although a large number of clamps can be installed along the fibre length for quasi-distributed operation. The advantages of this technique are in its response repeatabality.
5. Interferometric Techniques:
Interferometric fibre optic sensors are a large class of extremely sensitive fibre optic sensors. Fibre optic interferometers are analogous to their respective classic bulk optic interferometers. Fibre optic interferometers are generally intrinsic sensors in which light from a coherent source is equally divided to follow two (or more) fibre-guided paths. The beams are then recombined to mix coherently and form a “fringe pattern” which is directly related to the optical phase difference experienced between the different optical beams. This sensing technique is based primarily on detecting the optical phase change induced in the radiation field as it propagates along the optical fibre.
Fibre optic interferometers are typically used when ultra-high sensitivities are required and/or in applications of localised measurements (ie., point sensing), although sensor lengths longer than one meter are sometimes possible. Interferometers configured in a Mach-Zehnder or Sagnac configuration, however, enable truly distributed sensing to be performed. Furthermore, the Sagnac configuration makes it possible to locate a disturbance on the fibre system. Ultimately, the sensitivity and resolution of interferometers are limited by the effectiveness of the phase demodulation signal processing techniques used to interrogate the sensors.
The first types of fibre optic systems used for perimeter intrusion detection were based on destructive means, ie., the system relied on the optical fibre being cut, broken or severely bent in order to detect an event. Sometimes, these utilised OTDR to attempt to locate the events. These systems were found ineffective and inconvenient.
Truly modalmetric sensing systems, such as the SabreFonic from Pilkington P.E. Limited (UK) and Remsdaq Limited (UK), utilised the first non-destructive methods for perimeter monitoring. However, owing to the modulation of the modal pattern being non-linearly related to all disturbances, this method suffers from deep signal fading and drifting, resulting in many false alarms. For example, if the sun came out from behind clouds and suddenly warmed the fibre cable, the system response could be comparable or greater than the response from a true intrusion attempt. Consequently, this method suffers major problems from environmental conditions and is generally viewed as being quite unreliable.
In more recent years, advances in modalmetric techniques resulted in linear, more reliable systems, such as the Fiber Defender 200 Series (particularly the FD-220) from Fiber SenSys Inc. (USA) and the Foptic™ Secure Fence (FOSF™ from Future Fibre Technologies Pty. Ltd. (Australia).
The first system to be commercially available, the FD-220, offered considerable response and operational improvements from all previous systems. However, it still suffered from a number of limitations, as follows:                (a) According to promotional material, the maximum sensing length is limited to 1,000 to 2,000 meters. However, feedback from the industry is that the effective range is only 200 m.        (b) The system equipment for each zone needs to be mounted on the fence being monitored. This requires power to be provided externally to each system and results in a vulnerability to electromagnetic interference and lightning strikes. Furthermore, since the relatively short sensing range of the system requires a perimeter to be broken-down into a large number of zones for long perimeters, the system can be complex, expensive and subject to high maintenance requirements.        (c) It cannot pin-point the location of the disturbance.        
On the other hand, the FOSF™ is ideally suited to longer distance perimeters because of the nature of the unique sensing technique it employs. The FOSF™ can operate reliably over many tens of kilometres and theoretically over distances greater than two hundred kilometres. It can be operated as a single zone system using the Locator capability developed by Future Fibre Technologies Pty. Ltd. to identify any point of attempted intrusion, or it can be operated as a zoned system with zones of any desired length. A most important aspect of the FOSF™ configuration, zoned or using the Locator capability, is that no external electronics, optics or control hardware are required.
Currently, there is only one system employing the periodic microbending technique, the Inno-Fence from Magal Security Systems Limited (Israel). This system is based on a reliable sensing technique, but the requirement to clamp the fibre can lead to potential maintenance issues and the induced loss of light can severely limit the sensing range of the technique. Furthermore, the mechanical configuration of the system is quite limited and complex due to the need for the large number of clamping devices needed to cover the fibre length of interest. Consequently, this system is designed to monitor entire sections of panels in a picket-type fence configuration.
Interferometric fibre optic sensors, although offering very high sensitivity, and the ability to locate using a Sagnac configuration, are yet to offer an effective commercially available system to-date. This may be due to the very high sensitivity making the sensing device too sensitive/susceptible to environmental conditions and disturbances.
All but one of the above mentioned systems can be applied to virtually any type of perimeter barrier or fence, as well as being embedded in the ground. They can be used to protect such fence types as steel mesh and palisade, simply by attaching the fibre in a suitable manner to the fence. Most systems in use are based on these techniques.
However, the Inno-Fence system from Magal Security Systems Limited (Israel) is largely restricted to monitoring the panels situated between posts in a picket-type perimeter fence arrangement. This restriction stems from the requirement to incorporate the fibre clamping devices in the fence structure so that they are not visible and vulnerable to tampering. This had lead Magal to design quite a complex mechanical arrangement for the Inno-Fence system. Sensitivity of the system is to disturbance of the panel, not so much to each individual picket, because of the limited number of clamping devices and certain practical limitations to the physical configuration of the fence panel. These limitations and restrictions of the system results in problems with often inadequate sensitivity, the capability to overcome detection and quite serious maintenance issues.
Consequently, present inventor investigated and developed completely new methods for monitoring a perimeter barrier against intrusion or tampering utilizing the novel distributed fibre optic vibration sensing configuration detailed above. The novel distributed vibration sensing technique provides a fibre sensor which is highly sensitive to movement, displacement, loading and/or vibration of the fibre at any finite point along its length and does not require any particular physical configuration or fibre disturbing/clamping device to register an event. Consequently, much more convenient, effective, lower cost and aesthetic configurations for a picket-style fence are possible using this technique compared with what is available in the prior-art. The outcomes of this work are contained and claimed in this specification.
The main innovative features contained in the inventions disclosed in this specification are:
The systems operate using a novel distributed fibre optic vibration sensing technology or any other suitable, intrinsic distributed fibre optic sensor capable of detecting displacement, movement, loading and/or vibration of the optical fibre.
Each individual picket of the fence is attached to the distributed fibre optic vibration sensor and is thus sensitive to movement or physical disturbance.
A crossbar is not necessary to use with the pickets, although it is still possible to have one.
Movement sensitive panels can still be configured and utilised, with either the pickets or the supportive members instrumented to detect movement or physical disturbance.
The pickets or panels may be positioned between posts, or free-standing as in a palisade fence.
The monitoring systems are microprocessor based, situated in a central controls/alarm room and fully automated, providing real-time data analysis, logging and alarming features, and can be monitored and controlled locally or remotely.
Direct discussions with the industry have verified that there is very good commercial potential for the disclosed inventions and that there are clear advantages over the prior-art. It is important to note that the technology is considered to have good potential over competing techniques particularly because of the ease of sensor installation, the increased sensitivity and coverage, the excellent potential for system automation (ie., using cameras and remote communications) and the reduction in the required installation, operational and maintenance costs. Therefore, the inventions disclosed in this specification potentially offer lower cost products with enhanced capabilities and features.